1. Field of the Invention
This invention relates to locks with removable plugs and, more particularly, to a lock with a plug that is removable from and insertable into a cylinder with a change key and operable by an access key that is longer than the change key and directly interconnects the plug with a lock coupling.
2. Background of the Invention
A known lock construction employs a key-operated plug that is rotatably mounted in a lock cylinder. The plug has a plurality of combinating tumblers normally radially biased outwardly into a longitudinal slot associated with the cylinder to prevent rotation of the plug within the cylinder cavity. The combinating tumblers read a properly bited key in the keyway and retract to allow rotation of the plug in the cylinder cavity. The assembled plug cooperates with a rotatable coupling that is in turn associated with a slide lock, a bolt, or other conventional type locking structure commonly used to secure sliding drawers, hinged doors and the like on office furniture, cabinets and similar.
It is desirable to have the plugs removable and interchangeable for the following reasons. In operations having large numbers of drawers, doors and the like that are regularly locked, it may be desirable to have ccmmon keying though the furniture may not be purchased at the same time or from a common source. For this reason, manufacturers of furniture find it convenient to provide a cylinder that is compatible with lock plugs manufactured by any of a number of different sources. The consumer need only insert a desired plug without having to change the entire locking system. The manufacturers of the furniture systems can therefore avoid involvement in the actual lock mechanism construction.
Another situation that makes interchangeability of plugs desirable is where the tenure of employees or tenants with keys is terminated. By replacing just the plug, future access through the lock can be prohibited without completely changing the locking system.
It is known, as exemplified in U.S. Pat. No. 4,398,405, to Patriquin, to provide two separate keys, one of which is termed a change key to allow removal of the plug and the other of which is commonly referred to as an access key to normally operate the plug between locked and unlocked positions. In Patriquin, a control tumbler is provided and seats in a slot formed directly in the cylinder to prevent the plug from being withdrawn. To operate the control tumbler, the change key has an extra bited portion to engage and retract the control tumbler and thereby allow removal of the plug from the cylinder.
One problem with the Patriquin structure is that the control tumbler is a thin, spring loaded disc. The disc and spring are susceptible to being broken, in which event the lock would malfunction.
The Patriquin structure is at the same time susceptible to being picked. By forcibly wedging out the plug, the control disc might be severed and the plug removed, thereby permitting access to the locked structure. Alternatively, the control disc might be withdrawn by an individual extending a wire or the like into the keyway. Little security is afforded by the Patriquin structure.
The Patriquin structure also presents problems from a manufacturing end. The slot that accepts the control tumbler or the stop against which the control tumbler bears is directly formed in the cylinder. It is very difficult and thereby costly to cast this structure directly in the cylinder.
Further, the requirement of the additional moving disc and associated biasing structure unduly complicates assembly. This is particularly true when the parts are small as they are in the lock plug. Further, the number of parts necessarily increases the likelihood of malfunction. Any malfunction of the control tumbler would render the lock ineffective for its purposes.
A still further drawback with the Patriquin and other conventional prior art locking system is that the change key must be in place in the keyway to assemble the plug in the cylinder. In large operations several master change keys may have to be made available to installers. It may therefore be difficult to keep track and/or prevent duplication of these master change keys. Loss of security for an entire operation may be lost requiring replacement of all plugs.
Another prior art structure that is susceptible to picking is shown in U.S. Pat. No. 2,036,764, to Lowe. Lowe discloses a coupled plug and adapter which are movable as a unit by an access key that extends through the plug keyway into the adapter. The plug is rotatable relative to the adapter by inserting a change key that is sufficiently short that it does not penetrate the adapter. To interconnect the plug and adapter, the change key is inserted with the plug and adapter in the FIG. 9 orientation and the plug is rotated relative to the adapter until the plug and adapter are relatively situated as in FIG. 5.
With the plug rotated through the key to the FIG. 3, or locked position, one can pick the lock by shifting the locking bolt, which rotates along with the adapter freely relative to the fixed plug. Consequently, the security afforded by the Lowe lock is minimal.